And gallium and the next element in the periodic table, germanium, which was discovered about the same time, are important. Because in building up the theory of chemistry, the seminal idea is the periodic table of the elements. There were many people who put their mind to this and many partial developments, but the one which really hit the jackpot was Dmitri Mendeleev who, working in Leningrad, came up with this idea as a teaching aid, actually, just up… he came to it from his teaching of chemistry. He arranged the elements in the order of atomic weights. It’s been modified since, but that’s good enough for the present time. He did something more – he didn’t arrange them in a way which made chemical sense. He also said, the only way to make the known 63 elements, in those days, fit was to leave spaces otherwise the natural chemistry would not fall out. So he left spaces, and he did more. He said, this first space which he called eka-alluminium – ‘the next one after aluminium’ – is gallium. He predicted, did Mendeleev, what the properties of… and if you look at what Mendeleev predicted and what Lecoq de Boisbaudran found, it is just astonishing. The melting point, the liquid range, the valency, the chemical compounds, the stable oxide element... it was astonishing. And he did the same trick with germanium which was the next element in the periodic table that he’d left a gap for. And that really, more than anything else, was the seal of the periodic table: to be able to predict.

Norman Greenwood (1925-2012) was born in Australia and graduated from Melbourne University before going to Cambridge. His wide-ranging research in inorganic and structural chemistry made major advances in the chemistry of boron hydrides and other main-group element compounds. He also pioneered the application of Mössbauer spectroscopy to problems in chemistry. He was a prolific writer and inspirational lecturer on chemical and educational themes, and held numerous visiting professorships throughout the world.